Numerous systems are in use for underground distribution of alternating current (AC) electrical power and these systems utilize various types of cable constructions. For example, one type of cable construction in wide use in residential areas utilizes a central conductor, comprised of one or more strands, which is insulated by a comparatively thick tubular insulator system around which is supported an exposed concentric neutral conductor which may also be comprised of one or more strands. One of the problems with such a cable construction is soil, galvanic, and AC corrosion of the neutral conductor.
Another problem with underground cable constructions of the character mentioned is that the systems used heretofore utilize tinned copper to make the neutral conductor and with the comparative short supply and high cost of copper, it has become necessary to utilize other materials, such as aluminum, for both the neutral conductor and the central conductor. However, it is important to protect cable constructions whether made totally or partially of copper or totally or partially of aluminous materials against AC corrosion as well as soil and galvanic corrosion.
Regarding cathodic protection to prevent soil and galvanic corrosion in the United States, the National Association of Corrosion Engineers has established a recommended practice through a standard referred to as RP-01-69 that there should be a minimum negative polarization voltage shift of 150 millivolts for aluminum buried underground. Accordingly, one objective in an electrical cable construction for underground use is to provide cathodic protection that will produce a minimum negative polarization voltage shift of 150 mv.